Nothing will be better for complex music work than an 8-core Mac Pro. I admire your courage to realize the 4-core Mac Pro was more of a stop gap model than what the market needs longer term.
What's funny is that the 8-core Mac Pro will be more of a stop-gap model. After all, the Clovertown is two Woodcrest CPUs on the same die, but still running off the same FSB bandwidth and the first pair of cores must utilize the FSB to transfer data to the second pair of cores and vice versa. We won't see unified quad-core CPUs until sometime next year along with the multiplexed/bonded (and faster base rate) FSB implementations. ...AMD will be shipping fully unified quad-core CPUs in mid-December to early January. Not that it matters since Apple isn't using them.
Anyway, it's just another evolutionary step... Buy what you need when you need it and that's all there is to it.